The present invention is directed toward improving the power conversion efficiency of switch mode flyback power converters. A typical flyback converter includes a transformer with primary and secondary windings, a primary switch coupled in series with the primary winding, a rectifier coupled in series with the secondary winding, and an output capacitor coupled across the series combination of the secondary winding and rectifier. The primary switch is alternately closed (ON-period) and opened (OFF-period) within a fixed switching period. The rectifier and transformer windings are oriented such that power is coupled and stored in the transformer when the primary switch is closed, and thereafter released and coupled to the output capacitor when the primary switch is opened. In the latter state, the voltage across the transformer windings reverses, or "flies back", to release the energy and to reset the transformer's core.
Common applications for flyback converters are AC adapters, which may, for example, deliver an output voltage in the range of between 9 VDC to 24 VDC at power levels of 20 to 50 watts, drawing power from a rectified AC mains, which may vary between 85 VAC to 270 VAC. The flyback converter is preferred to other converters for these applications because it does not require an output choke, and because it can be designed to operate over a wide range of input voltages. However, the flyback converter has the disadvantage of poor power conversion efficiency, which raises the converter's power dissipation. The dissipation is an important consideration in that many such AC adapters are enclosed in plastic packages which do not readily dissipate the heat generated by the converter's power dissipation.
The relatively large primary leakage inductance of the primary winding is one factor that decreases the power conversion efficiency of the flyback in comparison to other converters. When the primary switch is closed, energy is stored in both the transformer's core (magnetizing inductance) and in the primary leakage inductance. When the primary switch is opened, the energy in the core (magnetizing inductance) is coupled to the secondary circuit, but the energy stored in the primary leakage inductance rings with the capacitance of the primary switch, and is conventionally dissipated in a voltage clamping or "snubber" circuit connected to the primary switch. This dissipated energy can easily be as much as ten percent (10%) of the energy transferred to the secondary circuit when using a safety isolated transformer and operating the flyback in discontinuous mode.
Another factor that decreases the power conversion efficiency is the relatively large root-mean-square (RMS) current that flows in the secondary winding during the OFF-period of the primary switch. As is known in the art, the current through the secondary winding during the OFF-period has a triangular waveform, starting at the beginning of the OFF-period substantially equal in value to the magnetizing current flowing in the primary winding at the end of the ON-period times the transformer turns ratio, and ending near the end of the OFF-period at zero amperes. Because the current waveform has a high initial current value, it has a relatively high RMS value in comparison to a flat current waveform transferring the same amount of energy. As is known in the art, the resistive losses (I.sup.2 R) in the secondary winding and secondary circuit are proportional to the square of the RMS current.
A further factor that decreases the power conversion efficiency of a conventional flyback converter are power dissipation losses that occur in the primary switch when it closes. The primary switch is normally implemented with a power field effect transistor (FET). Although such FET's can switch relatively quickly in comparison to bipolar power transistors, a measurable amount of power is dissipated in the FET when it is turned on (closed), since the drain voltage takes a finite time to decrease to near zero while drain current is flowing. Ideally, the turn-on losses could be reduced if the drain voltage of the FET could be reduced before current is conducted, i.e., zero-voltage switching.
The present invention is directed towards reducing the power dissipation in each of the above areas.